1. Field of the Invention
The present invention relates to a self-calibration method for sensors such as a displacement sensor, an angular sensor and so on.
2. Related Art
In developing displacement sensors and angular sensors on the order of nanometers and nano-radians, in addition to improving the accuracy thereof, it is difficult to obtain a calibration criterion that assures the accuracy thereof. Conventionally, to calibrate wavelength interpolation error of an interference displacement meter, a method using an X-ray interferometer (D. K. Bowen et al, "Subnanometer transducer characterization by X-ray interferometry", Precision Engineering, 12, 3 (1990) 165) and a non-linearity error compensating method using a linear drive range of a PZT (W. How and G. Wilkening, "Investigation and compensation of the non-linearity of heterodyne intergerometers", Precision Engineering, 14, 2 (1992) 91) have been proposed.
However, it is difficult for users to use the former method. On the other hand, in the latter method, it is difficult to determine whether or not calibrations have been correctly performed. In addition, most of such precision sensors require precise adjustments. Therefore, it is necessary to calibrate them in situ condition in such a manner that they are mounted in units. However, the conventional methods do not satisfy such needs.
To solve such a problem, the inventor of the present invention has proposed a self-calibration method for linearity errors of displacement sensors and angular sensors. In general, to obtain calibration data of a conventional sensor, a sensor system that has a higher accuracy than a sensor to be calibrated is required. In the self-calibration method, such an accurate sensor system is not required. The self-calibration that the inventor of the present invention has proposed are described in (1) Kiyono, Morishima, and Sugibuchi, "Self-calibration method against linearity errors for displacement meters", Journal of the Japan Society for Precision Engineering, 59, 12 (1993) 2043, (2) Kiyono, Ge, and Nishino, "High accuracy of interferometer by self-calibration of interpolation error", Journal of the Japan Society for Precision Engineering, 62, 2 (1996) 279, (3) Kiyono and Zhang, "Study of high accuracy self-calibration method for angular sensor", Journal of the Japan Society for Precision Engineering, 60, 11 (1994) 1591, and so forth.
In the self-calibration method for a displacement sensor, a reference sensor that is the same type of a sensor to be calibrated (hereinafter referred to as object sensor) is prepared. In addition, a lever system that allows the reference sensor to detect the n times the displacement of the object sensor is used. Thus, when the object sensor is calibrated with the reference sensor, the linearity error of the calibration result is decreased to 1/n by the lever system. By repeating the mutual calibrations, the linearity error can be converged to almost zero.
In the self-calibration method that the inventors of the present invention have proposed, the lever system that enlarges the calibration input values and the object sensor are required. Thus, it is difficult to calibrate a sensor in situ condition that the sensor is mounted in a unit. To accomplish such in situ calibration, a method that does not require an additional sensor and a lever system is desired.